Misc mypy in message_queue_node

Review/merge after !28 (merged)

I noticed that some of the code had type annotations, so I played around a bit with mypy. This fixes a few issues and adds a few annotations.

It's still not clean, but part of the beauty of mypy is that incremental annotations are still useful, so I figured I'd offer up a merge request in case y'all are interested =)

I ran mypy --ignore-missing-imports src/message_queue_node.py to check progress; it looks like it may have found an actual error around checking whether the message's fragment tracker is None before attempting to process an ack.